It is necessary to measure conditions in a drill hole, not only during the course of drilling, but after the drilling has been completed. While drilling, information is required about the condition of the rock being drilled (formation logging), about the drilling operation itself, for example, as to the weight acting on the drill bit, and the drill bit location, and about the physical conditions, for example, pressure and temperature, under which the drilling is taking place. When production has started, after drilling has been completed, information is required about flow amounts and conditions when the well is in production, together with information about the physical conditions at the rock formation whether the well is in production or not.
The necessary information is normally sensed by a logging unit which has to be located as close as possible to the position about which information is required. The unit may be installed together with a drill bit or may be sent down into a well by wireline techniques. The unit contains sensors and processing equipment for the sensor outputs. The data obtained may be recorded for reply when the logging unit is retracted, but is otherwise transmitted to the surface by a signal transmission system which may make use, as transmission conductors, of the earth, the drilling or production pipe, the drilling mud employed for drilling, or separate electric cables. The signals obtained in this way from the logging unit are received and decoded by a unit with appropriate detection and coding element.
The logging equipment should interfere as little as possible with extraction and drilling operations, and give accurate components and having a low cost. It must also have high reliability, with a service life of hundreds of hours, and any failure during operation should be easily rectified.
Normal conditions of operation however make it difficult to meet these requirements. Although well conditions vary considerably, temperatures of up to 150.degree. C. and pressures up to 1,000 bars are experienced. Vibrational loads up to 1,000 g can be experienced during drilling, and erosion and abrasion can result from varying qualities of drilling mud supplied along the interior of the rotating drill pipe. Power supply is a problem as batteries cannot provide adequate energy at the high temperatures experienced and wireline techniques are difficult to use where a pipeline deviates substantially from the vertical. Transmission rates also present difficulties because a large amount of data has to be carried, so a large data handling capacity is needed.